I have been to a few all-you-can-eat sushi restaurants and the catch at all of them was: it is only Nigiri (not sashimi) and you have to eat the rice.
I can't remember if they simply refused additional servings until the rice was gone or if they charged a la carte for un-eaten rice portions, but one way or another, the goal was you had to fill up on rice ...
If they start making the fish super-thin that's quite lame though. A buffet chain near me did that as they were trying, unsuccessfully, to survive. I also once went to a regular sushi places that had an all-you-can-eat special and who kept ("oopsie") accidentally bringing way more california rolls than I ordered.
My dad took us there a few times, he still talks about it and how sad he was when it closed down. He figures it just wasn't economically feasible. You got ridiculous amounts of really good, fresh food for fairly cheap. Far cheaper than ordering individually. We usually ended up eating until we all felt sick.
However, I would like to point out that this is a global phenomenon and I'd bet Finland has some of the least bad cases in this. Compare to some places like London where food is cheaper than Helsinki, but average salaries are higher. It's definitely about skirting regulations.
A interesting part that was left out from the initial post is that we have these "lunch coupons" in Finland. AFAIK most employers provide these for their employees either as part of the total compensation or allow the employees to buy them tax free. The idea is basically that you get lunch for the coupon, the max valued coupon is currently worth 10,70€. Seeing how more or less all restaurants that serves lunch accepts these coupons I don't think it is possible to charge much more than the max valued coupon.
On the other hand they must be getting fish dirt cheap. I have heard even the youth hostels serve entire salmon for breakfast.
BTW, its really bad for the environment because it encourages overfishing.
California roll 'crab' yes.
Usually the price you pay is 10€ for an unrestricted and no time limit access to eating sushi.
And the sushi is incredibly okay! It's nothing compared to stuff you find Tokyo ofcourse, but the price for quality is literally unbeatable.
And all this is happening in a city where restaurant prices for main dish are usually around 18-20€.
What am I missing?